A nuclear reactor has a core defining many parallel passages within which fuel elements and control elements are supported in different interrelated matrixes. The presence of the control elements relative to the fuel elements determines the reactivity beyond critical and thus the output of the reactor. Coolant is forced through the passages over the fuel and control elements for transferring the generated heat to heat exchanger means or the like whereby steam can be produced externally of the reactor and used to perform useful work.
One form of nuclear reactor, known as the liquid metal fast breeder reactor, uses molten sodium as the coolant; and in the pool type, the core is submerged in a pool of the sodium held in a reactor vessel. In the conventional pool type sodium cooled reactor, the core is supported from the reactor vessel, while the control elements are supported from a deck carried from and closing the top of the reactor vessel. This control element support arrangement is preferred for safety reasons, in that should the upper internal structures for holding the control elements fail and thereby drop the control elements further into the reactor core, their increased presence would reduce the reactivity of the reaction. On the other hand, there is also the possibility--much more remote--that the reactor vessel might fail and drop the core away from the control elements; which would increase the reactivity and possibly cause overheating damage to the core.
Of interest also, the reactor vessel must support all of the sodium coolant (which in a conventional 1000 MW electric reactor design can be in excess of 3,000 megagrams of sodium) and must also support the reactor core itself. The design must not only support these loads under static conditions, but must withstand seismic events, etc., which can generate appreciably greater dynamic loads. Consequently, the reactor vessel must be both durable and of high structural integrity.
In designing the reactor against seismic loading, one of the most important safety considerations is the relative motion of the reactor core and the control elements. For maximum safety, the movement of the reactor core should be minimized. If the reactor core is supported off the vessel side or bottom and the entire sodium inventory is brought into motion within the vessel, the flexibility of the vessel can allow the reactor core to move then relative to the reactor deck and the controls suspended therefrom. Even though the net movement of the control elements may be small compared to their total intentional movement, such can vary the control output of the reaction, both increasing and decreasing it. Also, this motion can induce stresses in the vessel and in the control structure.